Apparatus, method and computer program product for helicopter enhanced ground proximity warning system

ABSTRACT

An apparatus, method and computer program product for alerting rotary wing aircraft of potentially hazardous proximity to terrain also reduces nuisance warnings and provides a terrain display consistent with the unique performance capabilities of such aircraft.

CROSS REFERENCES TO RELATED APPLICATIONS

This application claims priority from U.S. Provisional application Ser.No. 60/207,515 titled “Terrain Floor Delta Height for Helicopter EGPWSBased on Ground Speed,” filed May 26, 2000; and from U.S. Provisionalapplication Ser. No. 60/207,740 titled “Terrain Awareness DisplayColoring for Helicopter EGPWS Based on Ground Speed,” filed May 26,2000; and from U.S. Provisional application Ser. No. 60/207,998 titled“Look Ahead Distance for Helicopter EGPWS Based on Stopping Distance,”filed May 26, 2000; the entire specifications of which are hereinincorporated by reference.

This application is also related to U.S. Provisional application Ser.No. 60/232,967, titled: “Tail Strike Algorithm for Helicopters” andfiled Sep. 14, 2000 and to application Ser. No. 09/865,365 filed thesame day herewith and titled: “Method, Apparatus and Computer ProgramProduct for Helicopter Tail Strike Warning”.

BACKGROUND OF THE INVENTION

The present invention provides a ground proximity warning system andmethod for rotary wing aircraft such as helicopters, gyrocopters, andtilt rotors and more particularly to logic and displays useful in ahelicopter enhanced ground proximity warning system, or EGPWS.

Ground proximity warning systems, or GPWS, provide aural and visualwarnings of conditions when the aircraft is in potentially hazardousproximity to terrain, and/or in a flight condition apparentlyinappropriate given the aircraft's position relative to terrain. Earliergeneration ground proximity warning systems sensed dangerous approach toterrain by using a radar altimeter to sense height above the ground. Therate at which height above ground changes, is compared with a predefinedenvelope(s) to determine if a dangerous condition exists. Classic GPWSsystems also contain additional alert functions called ‘modes’ thatalert to other potentially hazardous conditions based on flight regime.Examples of GPWS devices are contained in U.S. Pat. Nos.: 3,715,718;3,936,796; 3,958,218; 3,944,968; 3,947,808; 3,947,810; 3,934,221;3,958,219; 3,925,751; 3,934,222; 4,060,793; 4,030,065; 4,215,334; and4,319,218.

Later generation GPWS devices, called EGPWS devices or terrain awarenesssystems (TAWS), include a stored terrain database that compares theposition of the aircraft in three dimensional space with the storedterrain information to identify potential conflicts. EGPWS devices mayalso include all the functionality and modes of the classic GPWSdevices. Examples of EGPWS-type devices include U.S. Pat. Nos.:4,646,244; 5,839,080; 5,414,631; 5,448,563; 5,661,486; 4,224,669;6,088,634; 6,092,009; 6,122,570 and 6,138,060.

In certain EGPWS designs, the position of the terrain relative to theaircraft may be shown on a display in the cockpit. In some displays, theterrain is color-coded according to the degree of hazard. For example,green colored terrain usually depicts nonhazardous terrain below theaircraft. Yellow colored terrain usually depicts terrain that is inproximity to the aircraft and/or which may cause the ground proximitysystem to generate a precautionary alert. Red colored terrain usuallydepicts terrain at or above the aircraft altitude or for which theground proximity warning system will issue a warning from which evasiveaction must be taken. U.S. Pat. Nos.: 5,839,080 and 6,138,060 describesome terrain cockpit displays. U.S. Pat. No. 5,936,522 describes aterrain display having vertical and plan views.

The above referenced systems have been primarily developed for fixedwing aircraft. Rotary wing aircraft and aircraft capable of hoverpresent unique challenges for ground proximity alerting due to thedifferent flight profiles flown and the unique capabilities of rotarywing aircraft. For example, unlike fixed wing aircraft, rotary wingaircraft can cease all forward motion while still remaining airborne.Rotary wing aircraft can also descend straight down from a hover to landon all sorts of terrain, and need not make a gradual descent andapproach to land as in the case of fixed wing aircraft.

U.S. Pat. Nos. 5,781,126 titled “Ground Proximity Warning System andMethods for Rotary Wing Aircraft;” 5,666,110 titled “Helicopter EnhancedDescent After Take-off Warning for GPWS;” and 6,043,759 titled “AirGround Logic System and Method for Rotary Wing Aircraft;” and co-pendingapplication Ser. No. 08/844,116 titled: “Systems and Methods forGenerating Altitude Callouts for Rotary Wing Aircraft,” each addressvarious issues associated with applying ground proximity warningtechnology to rotary wing aircraft and are each incorporated herein byreference. These patents are applicable to both conventional andenhanced ground proximity warning designs for use in helicopters,however, these patents address the particularities of modifying variousof the “modes” for use in helicopters. Specifically, U.S. Pat. No.5,781,126 includes a barometric altitude rate detector including acontroller for adjusting this rate detector to account for downwash ofthe rotary wing. U.S. Pat. No. 5,666,110 discloses a descent aftertake-off protection envelope. U.S. Pat. No. 6,043,759 discloses a logicmethod and device for determining when the helicopter is in the airborneor ground state which assists with preventing nuisance alarms duringhelicopter autorotations. Ser. No. 08/844,116 discloses a device andmethod for generating altitude call outs during helicopter landingoperations.

None of the above mentioned patents account for modifying the terrainlook ahead logic or the associated terrain display of an EGPWS typedevice to account for the unique flying performance of helicopters andother rotary wing craft.

SUMMARY OF THE INVENTION

The present invention recognizes the problems in enhanced groundproximity alerting for rotary wing aircraft such as, for example,helicopters, gyrocopters and tilt rotors when in the rotor mode,hereinafter generically and interchangeably referred to as“helicopter(s)” or “rotary wing aircraft”. In particular, the presentinvention recognizes that certain categories of aircraft such as rotarywing aircraft and airships are capable of executing an avoidancemaneuver by coming to a stop in a hover. The present invention furtherrecognizes that the terrain threats as depicted on the display shouldpreferably also be modified to reflect the unique, yet normal operatingcapabilities of such aircraft. In addition, the present inventionrecognizes the unique landing characteristics of rotary wing aircraft.

According to one aspect of the present invention, a helicopter EGPWSuses a look ahead distance to define terrain that is a threat to theaircraft. If terrain is located within the boundaries of the warningenvelope, an alert is given to the pilot. In a preferred embodiment ofthe present invention, this look ahead distance is based upon thenominal distance required to halt forward momentum and enter a hover.

According to another aspect of the present invention, a helicopter EGPWSpermits the helicopter to land at any location, including off-airportlocations, without incurring nuisance alarms. In a preferred embodimentof the invention, the helicopter EGPWS uses groundspeed in conjunctionwith helicopter vertical speed to define a terrain floor below which aproximity warning will be given. In this manner, the helicopterexecuting a safe landing, for example, on a hillside will not incur anunwanted terrain proximity alert.

According to yet another aspect of the present invention, the presentinvention recognizes that a display color coded to alert pilots of fixedwing aircraft to potential hazards may result in displayingnon-threatening terrain as a hazard to pilots of rotary wing aircraft.The EGPWS and EGPWS display of the present invention color codes terrainbased upon flight regimes associated with modes of operation typical forrotary wing aircraft.

Further details and operation of the present invention will be describedbelow with reference to the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1B are a top level block diagram of an EGPWS computer for useon helicopters according to an embodiment of the present invention;

FIG. 2 is a functional block diagram of an EGPWS computer according toan embodiment of the present invention;

FIG. 3 diagrams six basic warning modes for a helicopter groundproximity warning system according to an embodiment of the presentinvention;

FIG. 4 is a side view of terrain caution and warning envelopes accordingto an embodiment of the present invention;

FIG. 5 is a perspective view of terrain caution and warning envelopesaccording to an embodiment of the present invention;

FIG. 6 illustrates a look ahead distance for use in determining terraincaution and warning envelopes for aircraft capable of hover according toan embodiment of the present invention;

FIG. 7 graphs a function useful for obtaining terrain floor ΔH foron-airport landings according to an embodiment of the present invention;

FIG. 8 graphs a function useful for obtaining terrain floor ΔH forhelicopters according to an embodiment of the present invention;

FIG. 9 is a graphical illustration of a cut-off angle correctionboundary for a level flight condition according to an embodiment of thepresent invention;

FIG. 10 is a graphical illustration of a cut-off angle correctionboundary for a condition where the flight path angle of the aircraft isgreater than a predefined reference or datum according to an embodimentof the present invention;

FIG. 11 is similar to FIG. 9 but for a condition where the flight pathangle of the aircraft is less than a predefined reference plane or datumand also illustrates a BETA sink rate enhancement boundary according toan embodiment of the present invention;

FIG. 12 is a graphical illustration of a cut-off altitude boundaryaccording to an embodiment of the present invention;

FIG. 13 illustrates a terrain caution and warning envelopes for acondition where the aircraft is descending according to an embodiment ofthe present invention;

FIG. 14 illustrates a terrain caution and warning envelopes for acondition where the aircraft is climbing according to an embodiment ofthe present invention;

FIG. 15 illustrates a look up caution and warning envelopes for acondition for detecting precipitous terrain ahead of the aircraftaccording to an embodiment of the present invention;

FIG. 16 is a functional block diagram for a system useful for assertinga signal representative of altitude loss due to pilot reaction time,ALPT, as well as altitude loss due to a pull up maneuver, ALPU,according to an embodiment of the present invention;

FIG. 17 is a functional block diagram of a terrain display systemaccording to an embodiment of the present invention;

FIG. 18 is a block diagram of a display system according to anembodiment of the present invention.

FIG. 19 illustrates depiction of background terrain on a displayaccording to an embodiment of the present invention;

FIG. 20 illustrates a background terrain display color and dot patterndensities according to an embodiment of the present invention; and

FIG. 21 illustrates an alternative background terrain display color anddot pattern densities according to an embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

System Overview

U.S. Pat. No. 5,839,080, incorporated herein by reference, describes anEGPWS device manufactured by Honeywell International Inc., and suitablefor use with the present invention. Additional EGPWS features applicableto rotary wing aircraft are described in U.S. Pat. Nos. 6,138,060;6,122,570; 6,092,009; 6,088,634; as well as in copending applicationSer. Nos.: 09/099,822; 09/074,953; 09/103,349; 09/255,670 and09/496,297, each of which is incorporated by reference. FIGS. 1A and 1Bprovide a top level description of such a system in block diagram form.The terrain awareness system 20 utilizes navigation information from aglobal positioning system 22 and/or a flight management system (FMS)and/or inertial navigation system. Navigation information may also bereceived from other aviation navigation systems such as, for example:DME/DME, VOR/DME, RNAV, and LORAN systems. System 20 further includes aterrain/obstacle database 24, and/or an airport database 26(collectively “terrain”) and a corrected barometric altitude signalwhich may be obtained from an air data computer or barometric altimeter28. In a preferred embodiment of the invention, altimetry informationcan be obtained in accordance with the techniques described inco-pending application Ser. No. 09/255,670 titled “Method and Apparatusfor Determining Altitude” and/or co-pending application No. titled,“Device, Method and Computer Program Product for Altimetry System” filedFeb. 2, 2001.

The latitude and longitude of the current aircraft position are appliedto an airport and terrain search algorithm, indicated by a block 29which includes location search logic for determining the terrain data,as well as the airport data surrounding the aircraft. Example searchlogic is described in U.S. Pat. Nos. 4,675,823 and 4,914,436 assigned tothe assignee of the present invention and incorporated herein byreference as well as in U.S. Pat. No. 5,839,080. The navigationalposition data, along with the terrain and airport data are supplied to athreat assessment function 30 which provides both terrain advisory andterrain warning signals based upon the position and flight path vectorof the aircraft. Function 30 may provide both aural and/or visualwarnings when a hazardous condition is believed to exist. Aural warningsmay be provided by voice generator 32 and speaker 34. Visual warningsmay be provided by a moving map or display 36. Display 36 may compriseany cockpit display, such as, for example, a weather radar display, aTCAS display, an Electronic Flight Instrument System (EFIS) display or aHoneywell UDI display. The terrain and obstacles depicted on display 36may be colored according to the degree of threat in a manner to bedescribed in greater detail below.

FIG. 2 contains a more detailed functional block diagram of EGPWScomputer 20 of the present invention. The EGPWS computer 20 as shown inFIGS. 1A-1B and in FIG. 2, may be implemented as executable code, ananalog or digital electronic circuit, on a PCMCIA card, as programmablelogic and/or as a general purpose processor. In a preferred embodimentof the invention, warning computer 20 is implemented as a linereplaceable unit (LRU) containing a microprocessor. Database 24 isincluded on a PCMCIA card which may be loaded into the LRU and also usedto provide periodic system upgrades.

In the embodiment of FIG. 2, local terrain processing function 29receives as input the aircraft position and heading data and retrievesfrom database(s) 24 and 26, the terrain, obstacle and/or runway data inthe vicinity of the aircraft. As described in U.S. Pat. No. 5,839,080,herein incorporated by reference for all purposes, terrain processingfunction 29 extracts and formats the local topographical data andterrain features to create a set of elevation matrix overlays that arepositioned with respect to the current aircraft location. Each matrixelement includes as data, the highest terrain altitude with respect tomean sea level contained within that element's area. Terrain processingfunction 29 optionally retrieves any obstacle data associated with thematrix elements as well as retrieving data for the runway nearest thecurrent location of the aircraft. Runway data includes the runwayendpoint locations and may be processed to include nearest runway centerposition, nearest runway threshold position, and nearest runwayaltitude.

The threat detection and terrain display processing function 30 receivesas input the terrain data as processed by function 29 as well as thecurrent aircraft position, altitude, attitude, speed and trackinformation. When a potential hazard to the aircraft exists, function 30controls the output of an alert which may include an aural warning, theillumination of a lamp, and/or depiction of the threat on display 36.Results of the threat detection process are combined with backgroundterrain data/obstacle matrix data and data for the nearest runway andformatted into a matching set of display matrix overlays for display ondisplay 36. A display output processor 50 receives the set of displaymatrix overlays from function 30 as well as the aircraft position andheading information to drive display 36. A display control logic 52controls the display range and the activation of display of terrain dataon the chosen display. In the embodiment of FIG. 2, display controllogic 52 controls an external switch 54 that may be used to switch aweather radar display from display of weather to display of terrain.

FIG. 3 diagrams the six basic warning modes for the helicopter groundproximity warning system of the present invention. The various modesprovide aural and visual alerts and warnings including warnings for:unsafe proximity to terrain, deviation below ILS glide slope, excessivebank angle, onset of severe wind shear, altitude awareness. Mode one,for example, provides pilots with alert/warnings for high descent ratesinto terrain. In this mode, a warning device compares the altitude aboveground of the aircraft with the descent rate, preferably barometricdescent rate, and issues a warning if the descent rate is excessive forthe altitude at which the aircraft is flying. A more completedescription of an exemplary warning device for indicating excessivedescent rate can be found in U.S. Pat. No. 4,551,723, the completedisclosure of which has previously been incorporated herein byreference. Mode two provides warnings for excessive closure rates toterrain with respect to altitude (AGL), phase of flight and speed. Modethree provides warnings for significant altitude loss after takeoff orlow altitude go around as described in U.S. Pat. No. 5,666,110. Acomplete description of the system can be found in U.S. Pat. No.4,818,992, the complete disclosure of which has previously beenincorporated herein by reference.

Mode four provides alerts and warnings for insufficient terrainclearance with respect to phase of flight and speed. Mode five providesglide slope alerts when the airplane is below 1,000 ft. AGL with thegear down and the glide slope deviation exceeds a threshold number ofdots below the ILS glide slope. Mode six provides callouts for descentthrough predefined altitudes (AGL). In particular, mode six is utilizedduring autorotation when the aircraft has lost all or partial enginepower. Each of the various warning modes provides at least an auralalert for a particular hazard as shown.

According to another embodiment of the invention, Mode 6 includes acapability for alerting the pilot of a helicopter of a possible tailstrike condition. Details of the Mode 6 tail strike alert are containedin copending U.S. application Ser. No. 09/865,365 filed the same dayherewith and incorporated herein by reference.

Terrain Caution and Warning Envelopes

In addition to the warning modes depicted in FIG. 3, and as describedabove in connection with FIGS. 1A-1B and FIG. 2, the EGPWS device of thepresent invention utilizes a plurality of caution and alert envelopes towarn of potential terrain hazards. If the aircraft penetrates thecaution envelope boundary, the aural message “Caution Terrain, CautionTerrain ” is generated, and alert discretes are provided for activationof visual annunciators. Terrain located within the caution envelope isshown in solid yellow color on display 36. If terrain proximate theaircraft penetrates the warning envelope boundary, the aural message“Warning Terrain” is generated, and alert discretes are provided foractivation of visual annunciators. Terrain located within the warningenvelope is shown in solid red color on display 36.

The caution and warning envelopes are obtained using a terrain floor anda “look ahead” distance to define a volume which is calculated as afunction of groundspeed and flight path angle. FIG. 4 shows a simplifiedside view of caution and warning envelopes 100 and 101 according to anembodiment of the present invention. FIG. 5 shows a perspective view ofthe caution and warning envelopes.

According to a preferred embodiment of the invention, terrain andcaution alerting can be provided for both a “normal” mode and a “lowlevel” mode of operation. The low level mode is applicable to lowaltitude flight in daylight VFR conditions. Use of the low level cautionand warning logic is selectable by the pilot and reduces nuisancewarnings by accounting for the unique low altitude operations of rotarywing aircraft. The normal mode comprises the default condition and isused for cruise, night time and instrument flight rules operations. Thepilot may select between the caution and warning envelopes of the normalmode and the alert envelopes and/or display of the low level mode via acockpit switch or display touch screen.

The look ahead distance of the caution and warning terrain protectionenvelopes is taken in a direction along the groundtrack of the aircraft.To reduce nuisance warnings, the look ahead distance may have a maximumvalue. Otherwise, potentially threatening terrain along the currentflight path of the aircraft relatively far from the current positioncould produce nuisance warnings. Two different look ahead distances(LAD) are utilized. The first LAD is used for a terrain caution signal.A second LAD is used for terrain warning signals which require immediateevasive action.

In aircraft not capable of hover, and in the normal mode, the LAD for aterrain advisory condition is considered first in determining the LADbecause it is assumed that the pilot could make a turn at any time at aturning radius R. For a fixed wing aircraft as fully described in U.S.Pat. No. 5,839,080, the total look ahead time is equal to the sum of thelook ahead time T₁ of a single turn of radius R; the look ahead time T₂for terrain clearance at the top of the turn plus a predeterminedreaction time T₃. In a helicopter, or other aircraft capable of hoverusing the low level mode, however, the look ahead distance can insteadbe based on the distance required to bring the aircraft to a stop orhover plus the distance covered during a nominal reaction time. In thecase of a helicopter, the distance required to transition from cruise tohover using, for example, a 10° pitch up at constant altitude may beused. FIG. 6 diagrams the look ahead distance for the helicopteraccording to the present invention.

The LAD can be expressed as:LAD=Transition Distance to Hover+Reaction Time Distance  Eq. (1).

Assuming the aircraft comes to a stop, the total distance covered whentransitioning to hover is governed by the standard equation:$\begin{matrix}{s = {\frac{1}{2}{at}^{2}}} & {{Eq}.\quad(2)}\end{matrix}$

where:

s=distance to stop

a=deceleration

t=time.

The average velocity during the deceleration interval t is:$\begin{matrix}{{\overset{\_}{V} = \frac{G\quad S}{2}},} & {{Eq}.\quad(3)}\end{matrix}$

where GS =groundspeed.

The distance covered can then be written as: $\begin{matrix}{s = {\frac{G\quad S}{2}t}} & {{Eq}.\quad(4)}\end{matrix}$

Substituting Eq. (4) back into Eq. (2); and solving for time, t, yields:$\begin{matrix}{t = \frac{G\quad S}{a}} & {{Eq}.\quad(5)}\end{matrix}$

Eq. 5 can then be used to develop an expression for stopping distance asgiven in Eq. (6) below: $\begin{matrix}{s = \frac{({GS})^{2}}{2a}} & {{Eq}.\quad(6)}\end{matrix}$

For a nominal pitch up of 10° the following equation can be used tosolve for a:ma=mg (tan 10°)  Eq. (7)

or, $\begin{matrix}{a = {{g\quad\tan\quad 10{^\circ}} = {{\left( {68682\quad\frac{nm}{{hr}^{2}}} \right)(0.18)} = {12363\quad\frac{nm}{{hr}^{2}}}}}} & {{Eq}.\quad(8)}\end{matrix}$

The above derivations have the advantage of making the calculation ofLAD independent of the aircraft mass and hence independent of aircrafttype.

For a predetermined reaction time T₃, for example, 10 seconds, the lookahead distance LAD in nautical miles for a terrain advisory signal canbe determined simply by multiplying the ground speed of the aircraft (V)by the reaction time T₃ and adding this value to the stopping distanceas shown in Eq. (9). $\begin{matrix}{{LAD} = {\left( \frac{\left( {G\quad S} \right)^{2}}{2a} \right) + {G\quad{S\left( T_{3} \right)}}}} & {{Eq}.\quad(9)}\end{matrix}$

Table I lists the resulting LAD for a 10° constant altitude transitionto hover and a reaction time of 5 seconds:

TABLE I LAD For 10° Pitch, Constant Altitude Hover And 5 Second ReactionTime GROUNDSPEED (kts) LAD (nm)  80 0.55 100 0.78 120 1.06

The LAD may optionally be additionally bounded by an upper limit and alower limit. The lower limit may be a configurable amount, for example;either 0.35, 1 or 1 ½ nautical miles at relatively low speeds e.g.speeds less than 40 knots, for example, and 4 nautical miles at higherspeeds, for example, greater than 250 knots. The LAD may also be limitedto a fixed amount regardless of the speed when the distance to thenearest runway is less than a predetermined amount, for example, 2nautical miles, except when the aircraft altitude is greater than 500feet, relative to the runway.

In a preferred embodiment of the invention, the LAD for a terrainwarning signal is taken as ½ the terrain caution signal LAD. Optionally,and as described more fully in U.S. Pat. No. 5,839,080, the terrainwarning LAD may be given by equation (10) below.LAD=k₁*LAD (terrain caution), k₂*LAD (terrain warning), K₃ LAD (terrainlook-up advisory)  Eq. (10)

where k₁=1.5, except when the LAD is limited at its lower limit, inwhich case k₁=1, k₃=1 and where k₂=0.5, k₃=2.

As shown in FIGS. 4 and 5, the caution and advisory envelopes areadditionally defined by a terrain floor boundary. In a fixed wingaircraft, the terrain floor relates to a distance AH below the aircraftand is proportional to the distance to the closest runway to preventnuisance warnings when the aircraft is taking off and landing, whileproviding adequate protection in other modes of operation. FIG. 7illustrates the terrain floor AH used for airport landings. The terrainfloor boundary below the aircraft is essentially based upon providing100 feet clearance per nautical mile from the runway as identified byrunway centerpoint 72 and endpoint 80, limited to 150 feet. In apreferred embodiment of the invention, distance D from runwaycenterpoint 72 is equal to 12 nm.

However, in aircraft capable of landing safely off-airport, such as ahelicopter, nuisance alarms will occur when the helicopter lands on, forexample, a hillside or other safe but off-airport/helipad location. Forthis reason, the present invention utilizes the terrain floor ΔH of FIG.8. In FIG. 8, the horizontal axis represents the groundspeed while thevertical axis represents the ΔH terrain floor boundary beneath theaircraft. The terrain floor ΔH boundary beneath the aircraft is limitedsuch that the segment commencing at point 78 beings at 0 feet and thesegment 82 never goes above a predetermined maximum, for example, 150feet. The groundspeed corresponding to point 78 is preferably theairspeed corresponding to the landing or touchdown speed in zero windconditions. Therefore, as illustrated in FIG. 8, so long as the pilotcontinuously slows the aircraft while descending, no terrain alert willbe given by the present invention. Such conditions are indicative of anapproach to land and are not likely to be indicative of a controlledflight into terrain accident.

For helicopters with retractable landing gear, the terrain floor deltaheight function of FIG. 8 can be additionally coupled to logic thatdetects when the gear is deployed.

If the gear is not deployed, the curve of FIG. 8 can be disabled and theΔH curve of FIG. 7 utilized. The Mode 4 “Too Low Gear” warning will alsosound.

In order to avoid spurious warnings when the aircraft over flies a ridgeat relatively low altitudes, the terrain advisory and warning boundariesmay additionally include cut-off boundaries, for example, as illustratedin FIGS. 9, 10 and 11. Without the cut-off boundaries, warnings would begiven, although the terrain is virtually below the aircraft and noterrain is visible ahead. In FIG. 9, the cut-off boundary 126 begins ata predetermined cut-off offset 128 below the aircraft and extends in adirection in front of the aircraft at a predetermined envelope cut-offangle 130. The envelope cut-off angle 130 is equal to the flight pathangle y plus a configurable predetermined cut-off angle, described andillustrated as 6°. For level flight as shown in FIG. 9, the cut-offboundary 126 extends from the cut-off offset 128 in the direction of theenvelope cut-off angle 130 toward the front of the aircraft to a point132 where it intersects a terrain caution boundary or terrain warningboundary, identified with the reference numeral 134. For level flight,as shown in FIG. 9, the flight path angle y is zero. Thus, the cut-offboundary 126 illustrated in FIG. 9 will extend from the cut-off offset128 along an angle equal to the cut-off angle.

The cut-off boundary 126 extends from the cut-off offset 129 to thepoint 132 where it intersects the terrain caution boundary 134. Thewarning boundary is then selected to be the highest of the terraincaution boundary 134 and the envelope cut-off boundary 126. Thus, forthe example illustrated in FIG. 9, the terrain caution boundary wouldinclude the cut-off boundary 126 up to the point 132, where the envelopecut-off boundary 124 intersects the warning envelope 126. From the point132 forward, the normal terrain caution boundary 134, corresponding, forexample, to a THETA1 slope is utilized. Thus, if either a terraincaution or terrain warning boundary is below the cut-off boundary 126,the cut-off boundary 126 becomes the new boundary for the caution orwarning signal.

The cut-off boundary may additionally include a cut-off altitude. Thecut-off altitude is an altitude relative to the nearest runwayelevation; set at, for example, 500 feet. Altitudes below the cut-offaltitude are ignored by the terrain advisory and terrain warningcomputations. An advantage to using a cut-off altitude is that nuisancewarnings on final approach, due to altitude errors, terrain databaseresolution and accuracy errors are minimized.

However, the use of a cut-off altitude during certain conditions, suchas an approach to an airport on a bluff (i.e. Paine Field) at arelatively low altitude or even at an altitude below the airportaltitude, may compromise system performance. More particularly, duringsuch conditions, the use of a cut-off altitude may prevent a terrainwarning from being generated because the threatening terrain may bebelow the cut-off altitude. In order to account for such situations andfor the ability of helicopter type aircraft to land safely off theairport, the system selects the lower of two cut-off altitudes; anearest runway cut-off altitude (NRCA) and a cut-off altitude relativeto aircraft (CARA). The NRCA is a fixed cut-off altitude, relative tothe nearest runway. The CARA is an altitude below the instantaneousaircraft altitude (ACA) by an amount essentially equivalent the ΔHterrain floor boundary of FIG. 7 or 8, whichever is smaller.

Equations (11) and (12) below set forth the NRCA and CARA. As mentionedabove, the absolute cut-off altitude (ACOA) is the lower of the NRCA andCARA as set forth in equation (13).NRCA=COH+RE,  Eq. (11)

where COH relates to the cut-off height and is a fixed configurablevalue, initially set between 400 feet and 500 feet; and RE equals therunway elevation.CARA=ACA-ΔH-DHO,  Eq. (12)

where ACA is the instantaneous aircraft altitude; ΔH is the smaller ofthe terrain floor of FIG. 7 or 8 and DHO is a configurable bias, set to,for example, 50 feet.ACOA=lower of CARA, NRCA,  Eq. (13)

For landings in the vicinity of an airport runway contained in thedatabase, however, a point, DH1, exists for which the ACOA is forced tobe equal to NRCA independent of the aircraft altitude. The point DH1 isrelated to COH, AΔand DHO such that on a nominal three (3) degreeapproach slope, CARA is equal to NRCA when the aircraft is at a distanceequal to a distance DH1 from the airport, as illustrated in Table IIbelow:

TABLE II Relationship Between COH, DH1 and Runway Distance COH DH1DISTANCE TO RUNWAY (feet) (feet) (n mile) 300  50 1 400 100 1.5 500 1502

The point DH1 forces the cut-off altitude (COH) above the runwaywhenever the aircraft is close to the runway to ensure robustnessagainst nuisance warnings caused by altitude errors and terrain databaseresolution margins to disable the terrain caution and warningindications when the aircraft is within the airport perimeter. There aretrade-offs between nuisance warnings and legitimate warnings. Inparticular, the lower COH, the closer the caution and warningindications are given, making the system more nuisance prone. Asindicated above, for a COH of 400, terrain caution and warningindications are effectively disabled when the aircraft is closer than1.5 nm from an airport runway.

FIG. 12 illustrates the operation of the alternative cut-off altitudeboundaries. In particular, FIG. 12 illustrates a condition when the COHis set to 300 feet with DH1 equal to 50 feet. The cut-off altitude foran area from the runway, for example, greater than 4 nautical miles, is300 feet, as indicated by the segment 278 when the glide slope angle isless than a predetermined angle, for example, 3°. As the aircraft getscloser to the runway, the COH is lowered, as illustrated by the segment288, until the aircraft is within one (1) nautical mile of the runway,at which point the COH is forced to be 300 feet, which effectivelydisables any terrain caution and warning indications when the aircraftis closer than one (1) nautical mile from the runway, as represented bythe segment 282. During a condition when the aircraft is on, forexample, a 3° glide slope angle to the database runway, the ACOA isforced to be the NRCA. The NRCA is illustrated by the segment 282 ofFIG. 12.

The resulting terrain threat envelopes are illustrated in FIGS. 13-15.The terrain caution and warning envelopes may be thought of as includingtwo parts: a look-ahead/look-down boundary for detecting terrain aheador below the aircraft as shown in FIGS. 13 and 14; and a look-upboundary for detecting precipitous high terrain ahead of the aircraftwhich may be difficult to clear as shown in FIG. 15.

In FIG. 13, caution and warning envelopes are illustrated for acondition when the aircraft is descending (i.e. γ<0). During such acondition, the first segment of the terrain caution envelope, identifiedwith the reference numeral 300, corresponds to the ΔH terrain floorboundary. To determine the bottom segment 302 of the terrain cautionenvelope, the flight path angle y is compared with a configurable datum,THETA1, for example 0°. During descent conditions, the flight path angley will thus be less than THETA1. Thus, the look-ahead/look-down terrainadvisory boundary segment will extend from the ΔH terrain floor boundarysegment 300 along the angle THETA1 to the look-ahead distance for aterrain advisory (LAD). The final segment 304 extends vertically upwardfrom the segment 302 along the LAD.

The terrain caution boundary may also be modified by a BETA sink rateenhancement. In this embodiment, the BETA sink rate enhancement ensuresthat an advisory indication always precedes a warning indication whenthe aircraft descends into or on top of terrain. The BETA sink rateenhancement is determined as a function of the flight path angle γ andtwo (2) configurable constants KBETA and GBIAS. The BETA sink rateenhancement BETA1 for a look-ahead/look-down terrain advisory isprovided in equation (14) below for a condition when the aircraft isdescending. The angle BETA1 is given by:BETA1=KBETA*(γ-GBIAS),  Eq. (14)

where GBIAS is a configurable constant, selected for example, to be zero(0) and KBETA is also a configurable constant selected, for example, tobe 0.5. In the embodiment of FIG. 13, the BETA sink rate enhancementBETA1 for the look-ahead/look-down terrain advisory boundary provides anadvisory warning at a distance of ½ LAD. The BETA sink rate enhancementBETA1 results in a segment 306 which extends from the ΔH terrain floorsegments 300 at an angle equal to γ/2 up to ½ of the LAD. Beyond ½ LAD,a LAD, a segment 308 extends at the angle THETA1 to a distance equal tothe LAD. A vertical segment 310 extends along the LAD to connect thesegments 308 to the segment 304.

In FIG. 13, the cut-off boundary is identified with the referencenumeral 312. The cut-off boundary 312 extends from a vertical distance314 below the aircraft along a cut-off angle up to the point ofintersection 316 with the terrain advisory boundary. For distances lessthan the intersection 316, the cut-off boundary 312 forms the terraincaution envelope boundary. For distances beyond the point ofintersection 316, the boundaries 306 and 308 form the terrain cautionboundaries.

The terrain warning boundary includes the segment 300 extending from theaircraft along the ΔH terrain floor. A bottom segment 318 connects tothe segment 300 and extends along a BETA sink rate enhancement angleBETA2, where angle BETA2 is given by the equation:BETA2=KBETA2*(γ-GBIAS),  Eq. (15)

where GBIAS is a configurable constant selected, for example, to be 0and KBETA2 is also a configurable constant selected, for example, to be0.25. For such values of the constants KBETA2 and GBIAS, the BETAenhancement angle KBETA2 will be ¼*γ. Thus, the segment 318 extends fromthe segment 300 at an angle equal to ¼*γ up to ½ the LAD. A verticalsegment 320 extends along a distance equal to ½*LAD from the segment 318to define the terrain warning boundary.

The terrain warning boundaries are also limited by the cut-off boundary312. Thus, the cut-off boundary 312 forms the terrain warning boundaryup to a point 322, where the cut-off boundary 312 intersects the lowerterrain warning boundary 318. At distances beyond the point ofintersection 322, the segment 318 forms the lower terrain warningboundary up to a distance equal to ½ of the LAD.

The terrain advisory and terrain warning boundaries for a condition whenthe aircraft is climbing (i.e. γ>0) is illustrated in FIG. 14. Duringsuch a condition, the BETA sink rate enhancement angles BETA1 and BETA2are set to a configurable constant, for example, zero (0). The terrainadvisory boundary during the climbing condition is formed by extending avertical segment 324 from the aircraft for a distance below the aircraftequal to the ΔH terrain floor of FIG. 7 or 8, whichever is smaller. Asegment 326 is extended from the segment 324 to the LAD at an angleequal to the flight path angle γ. At a point 328 where the segment 326intersects a position equal to ½ of the LAD, a vertical segment 330 isextended up from the segment 326, forming a first vertical boundary forthe terrain advisory condition. The line segment 326 from the point 328to the LAD forms the lower terrain advisory boundary while a linesegment 332 extending vertically upward from the line segment 326 alongthe LAD forms a second vertical boundary.

For the exemplary condition illustrated, a cut-off boundary 334 does notintersect the terrain caution boundaries. Thus, the terrain cautionboundaries for the exemplary condition illustrated is formed by thesegments 330 and 332 and that portion of the line segment 326 betweenthe line segments 330 and 332.

The terrain warning boundaries for the climbing condition of FIG. 14include the vertical segment 324 which extends from the aircraft tovertical distance equal to the ΔH terrain floor of FIG. 7 or 8,whichever is smaller, below the aircraft forming a first verticalboundary. For a condition when the aircraft is climbing, the linesegment 326 extends from the segment 324 at the flight path angle γ toform the lower terrain warning boundary. The vertical segment 330 at adistance equal to ½ of the LAD forms the second vertical terrain warningboundary.

The cut-off boundary 334 limits a portion of the terrain warningboundary. In particular, the cut-off boundary 334 forms the lowerterrain warning boundary up to a point 340, where the cut-off boundary334 intersects the line segment 326. Beyond the point 340, a portion 342of the line segment 340 forms the balance of the lower terrain warningboundary up to a distance equal to ½ of the LAD.

A look-up terrain advisory and terrain warning boundaries areillustrated in FIG. 15 and are applicable for the condition where theaircraft is not slowing for an off-airport landing. As will be discussedin more detail below, the look-up terrain advisory and warningboundaries start at altitudes DHYEL2 and DHRED2, respectively, below theaircraft. These altitudes DHYEL2 and DHRED2 are modulated by theinstantaneous sink rate (i.e. vertical speed, HDOT of the aircraft). Theamount of modulation is equal to the estimated altitude loss for apull-up maneuver, for example, at ¼ G (e.g 8 ft/sec²) at the presentsink rate. The altitudes DHRED2 and DHYEL2 are dependent upon thealtitude loss during a pull-up maneuver (ALPU) and the altitude loss dueto reaction time (ALRT) as given by:ALRT=HDOT*T_(R),  Eq. (16)

where HDOT equals the vertical acceleration of the aircraft in feet/sec.and T_(R) equals the total reaction time of the pilot in seconds.

Assuming a pull-up maneuver is initiated at time T_(I), the altitudeloss due to the pull-up maneuver ALPU may be determined by integratingthe vertical velocity HDOT with respect to time as set forth below.HDOT(t)=α*t+HDOT₀  Eq. (17)

where “a” equals the pull-up acceleration and HDOT.₀ is a constant.

Integrating both sides of equation (17) yields the altitude loss as afunction of time H(t) as provided in equation (18) below.$\begin{matrix}{{H(t)} = {{\frac{1}{2}a\quad t^{2}} + {\left( {HDOT}_{0} \right)t}}} & {{Eq}.\quad(18)}\end{matrix}$Assuming a constant acceleration during the pull-up maneuver, the time tuntil vertical speed reaches zero is given by equation (19).$\begin{matrix}{t = \frac{- {HDOT}_{0}}{a}} & {{Eq}.\quad(19)}\end{matrix}$

Substituting equation (19) into equation (18) yields equation (20).$\begin{matrix}\frac{{HDOT}_{0}^{2}}{2a} & {{Eq}.\quad(20)}\end{matrix}$

Equation (20) thus represents the altitude loss during the pull-upmaneuver.

An exemplary block diagram for generating the signals ALRT and ALPU isillustrated in FIG. 16. In particular, a signal representative of thevertical velocity of the aircraft HDOT, available, for example, from abarometric altimeter rate circuit (not shown), is applied to a filter350 in order to reduce nuisance warnings due to turbulence. The filter350 may be selected with a transfer function of 1/(TAUDOT*S+1); whereTAUDOT is equal to one second. The output of the filter 350 is a signalHDOTf, which represents the filtered instantaneous vertical speed;positive during climbing and negative during descent.

To obtain the altitude loss due to reaction time signal ALTR, the signalHDOTf is applied to a multiplier 352. Assuming a pilot reaction timeT_(r), for example, of 5 seconds, a constant 354 equal to 5 seconds isapplied to another input of the multiplier 352. The output of themultiplier 352 represents the signal ALTR, which is positive when HDOTfis negative and set to zero if the signal HDOTf is positive, whichrepresents a climbing condition. More particularly, the signal HDOTf isapplied to a comparator 356 and compared with a reference value, forexample, zero. If the comparator 356 indicates that the signal HDOTf isnegative, the signal HDOTf is applied to the multiplier 352. Duringclimbing conditions, the signal HDOTf will be positive. During suchconditions, the comparator 356 will apply a zero to the multiplier 352.

The altitude loss due to the pull-up maneuver signal ALPU is developedby a square device 358, a divider 360 and a multiplier 362. The filteredinstantaneous vertical speed signal HDOTf is applied to the squaredevice 358. Assuming a constant acceleration during the pull-upmaneuver, for example, equal to 8 feet/sec² (0.25 _(g)), a constant isapplied to the multiplier 362 to generate the signal 2 a. This signal, 2a, is applied to the divider 360 along with the output of the squaredevice 350. The output of the divider 360 is a signal (HDOTf) 2/2a,which represents the altitude loss during a pull-up maneuver signalALPU.

These signals ALRT and ALPU are used to modulate the distance below theaircraft where terrain advisory and terrain warning boundaries beginduring a look-up mode of operation. More particularly, during such amode of operation, the portion of the ΔH terrain floor segment of theterrain caution envelope contributed by DHYEL2 is modulated by thesignals ALRT and ALPU while the ΔH terrain floor of the terrain warningboundary DHRED2 is modulated by the signal ALPU as indicated inequations (21) and (22), respectively.DHYEL2=¾*ΔH+ALRT+ALPU  Eq. (21)DHRED2=½*Δ+ALPU,  Eq. (22)

where ΔH represents the terrain floor of FIG. 8? as discussed above.

Thus, in FIG. 15, the look-up terrain caution boundary begins at a point364 below the aircraft; equal to DHYEL2. If the flight path angle γ isless than a configurable datum THETA2, a terrain advisory boundary 366extends from the point 364 to the advisory LAD at an angle equal toTHETA2. Should the flight path angle y be greater than THETA2, the loweradvisory boundary, identified with the reference numeral 368, willextend from the point 364 at an angle equal to the flight path angle γ.

Similarly, the look-up terrain warning boundary begins at point 370below the aircraft; equal to DHRED2. If the flight path angle γ is lessthan the angle THETA2, a warning boundary 372 extends from the point 370at angle THETA2 to the warning LAD. Should the flight path angle γ begreater than THETA2, a warning boundary 374 will extend at an angleequal to the flight path angle γ between the point 370 and the warningLAD.

Terrain Display

A display system, generally identified with the reference numeral 400,is illustrated in FIGS. 17 and 18. FIG. 17 contains a top levelfunctional diagram of the warning computer/display interface, while FIG.18 represents a simplified block diagram for implementation of thedisplay system 400 in accordance with the present invention. The displaysystem 400 may include a microprocessor, for example, an Intel type80486, 25 MHz type microprocessor (“486”) and an Analog Devices typeDigital Signal Processor (DSP). The DSP is primarily used forcalculating the RHO/THETA conversions to off load the 486 microprocessoror to facilitate display of terrain delta on a display device typicallyused to display weather radar data in a RHO/THETA format. Additional,details on the construction of the terrain data display overlaymatrices, terrain data files and display drivers may be found in U.S.Pat. No. 5,839,080 which has been incorporated herein by reference.

The display system 400 is used to provide a visual indication of terrainwhich penetrates the terrain caution and terrain warning envelopes aswell as for display of terrain in the current vicinity of the aircraft.The background terrain information may be displayed using dot patternswhose density varies as a function of the elevation of the terrainrelative to the altitude of the aircraft and color coded according tothe degree of threat. Terrain located within the caution and warningenvelopes may be displayed in solid colors, such as yellow and red.

FIG. 19 illustrates how the terrain background information can be shownon the display 36. As will be discussed in more detail below, theelevation of the terrain relative to the altitude of the aircraft isshown as a series of colored dot patterns whose density varies as afunction of the distance between the aircraft and the terrain. Thecolors are used to distinguish between terrain caution and terrainwarning indications. For example, red may be used to represent a terrainwarning indication while yellow or amber is used to represent a terrainadvisory indication. By using colored shapes for terrain threatindications and dot patterns of variable density for the terrainbackground information, clutter of the display is minimized.

FIG. 20 illustrates display 36 color and dot pattern densities suitablefor use with helicopters and other aircraft capable of off-airportlandings. Terrain located more than 1500 feet below the aircraft is notshown. Terrain located between 500 and 1500 feet below the aircraft isshown in green with a dot pattern density of 16%. Terrain less than 500feet below the aircraft but greater than a predetermined amount beneaththe aircraft is shown in green with a dot pattern density of 50%.

The predetermined amount defines the boundary between where terrain isshown as green, indicating non-threatening terrain, and where terrain isshown as yellow indicating a potential hazard. According to the presentinvention, this predetermined value is ascertained as a function of theaircraft phase of flight. For example, during cruise, the yellow/greenboundary is located 250 feet below the aircraft. The cruise phase can beidentified by monitoring the groundspeed of the aircraft. In a preferredembodiment of the invention, cruise is identified as a condition wherethe groundspeed is equal to or exceeds 90 kts. During approachconditions, the yellow/green boundary is located at the aircraftaltitude. In a preferred embodiment of the invention, groundspeedsbetween 40 and 90 kts are indicative of the approach condition. Duringhover or landing conditions, the yellow/green boundary is located at thecurrent aircraft altitude. In a preferred embodiment of the invention,the hover condition is defined as a groundspeed below 40 kts.Preferably, the groundspeeds defining the hover and cruise conditionsalso form corner points 78 and 82 of the ΔH curve of FIG. 8. Terraindepicted in yellow above the yellow/green boundary is shown with a dotdensity of 25%. For terrain elevations located between 250 feet and 500feet above the aircraft current altitude, the background terrain iscolored yellow using a 50% dot density. Terrain elevations greater than500 feet above the current aircraft elevation are colored red using a50% dot density. Terrain located in the caution advisory envelope iscolored solid yellow, while terrain located within the warning envelopeis colored solid red. Bodies of water located at sea level mayoptionally be colored in cyan, or other shade of blue.

FIG. 21 illustrates an alternative terrain display suitable for use withthe present invention.

The present invention thus enables those aircraft capable of routine,safe, off-airport landings to land at a location other than an airportwithout flying through terrain that would otherwise be depicted asyellow or red on a terrain display for fixed wing aircraft. Overflyingterrain that is depicted as yellow or red during off airport landings,can lead to complacency, and the pilot may ignore such a display in thefuture believing a safe condition exists when in reality it does not.The present invention thus permits the background terrain display toreflect a safe approach to an off airport landing by matching thedisplay of potentially hazardous terrain to phase of flight. The presentinvention thus also preserves the depiction of terrain as hazardous, andassists in the prevention of CFIT accidents, when the approach andlanding conditions do not exist and the pilot is in hazardous proximityto terrain.

The invention has now been described with reference to the preferredembodiments. Variations and modifications will be readily apparent tothose of skill in the art. For this reason, the invention is to beinterpreted in view of the claims.

1. An apparatus for alerting a pilot of a rotary wing aircraft ofproximity to terrain, the apparatus comprising: an input for receivingsignals representative of a position of the aircraft, a flight pathangle of the aircraft and a speed of the aircraft, and coupled to adatabase of stored terrain information; an output; a signal processingdevice, coupled to said input, and coupled to said output, for: (a)defining a look ahead distance; (b) defining a first alert envelope,indicative of a first severity of terrain threat, wherein boundaries ofsaid first alert envelope are determined as a first function of theflight path angle, said look ahead distance, and a terrain floorboundary; wherein said terrain floor boundary comprises a function of anaircraft altitude and said speed; (c) defining a second alert envelope,indicative of a second severity of terrain threat, wherein boundaries ofsaid second alert envelope are determined as a second function of theflight path angle, said look ahead distance and said terrain floorboundary; and (d) outputting an alert signal when a subset of the storedterrain information is located within the boundaries of at least one ofsaid first and said second alert envelopes.
 2. The apparatus of claim 1wherein at least one of said first and second alert envelopes is furtherbounded by a cut-off envelope.
 3. The apparatus of claim 1 wherein saidsignals representative of the position of an aircraft include a firstsignal received from a satellite navigation system indicative of theaircraft altitude and a second signal representative of the aircraftaltitude received from a source other than the satellite navigationsystem, and wherein said signal processing device further comprises ameans for determining a compound altitude signal.
 4. The apparatus ofclaim 1 wherein the boundaries of at least one of said first and secondalert envelopes is further determined as a function of a configurabledatum.
 5. The apparatus of claim 1 wherein at least one of said firstand second alert envelopes further comprises a subset of alert envelopesrepresenting various severities of hazard to the aircraft.
 6. Theapparatus of claim 1 wherein said signal processing device comprises amicroprocessor.
 7. The apparatus of claim 1 wherein said signalprocessing device comprises a means for outputting said alert signal asa video control signal, wherein said video control signal is useful forcontrolling representation of terrain on a video display in variouscolors according to a degree of terrain threat.
 8. The apparatus ofclaim 1 further comprising a voice warning generator coupled to saidsignal processor and wherein said alert signal output from said signalprocessing device comprises an audio control signal to command saidvoice warning generator to output an aural alert.
 9. The apparatus ofclaim 1 wherein said speed comprises a groundspeed of the aircraft. 10.The apparatus of claim 1 wherein the aircraft is a tilt rotor.
 11. Theapparatus of claim 1 wherein said signal processing device furthercomprises a means for outputting a video control signal to controlrepresentation of a background terrain data proximate the aircraft: in afirst color for terrain located more than a predetermined amountrelative to a current altitude of the aircraft wherein saidpredetermined amount is a first value for a cruise phase of flight and asecond value for an approach phase of flight and a third value for alanding phase of flight; and in a second color for terrain located lessthan said predetermined amount relative to said current altitude. 12.The apparatus of claim 11 wherein said cruise, approach and landingphases are defined as a function of said speed of the aircraft.
 13. Theapparatus of claim 1 wherein said look ahead distance is a function of adistance to transition from a first phase of flight to a hover phase offlight.
 14. A method for alerting a pilot of a rotary wing aircraft ofproximity to terrain comprising the steps of: accessing a database ofterrain information; receiving signals representative of a position ofthe aircraft, a flight path angle of the aircraft and a speed of theaircraft; defining a look ahead distance; defining a first alertenvelope, indicative of a first severity of terrain threat, whereinboundaries of said first alert envelope are determined as a firstfunction of the flight path angle, said look ahead distance, and aterrain floor boundary; defining a second alert envelope, indicative ofa second severity of terrain threat, wherein boundaries of said secondalert envelope are determined as a second function of the flight pathangle, said look ahead distance and said terrain floor boundary;defining said terrain floor boundary as a function of an aircraftaltitude and said speed; and outputting an alert signal when a subset ofthe stored terrain information is located within the boundaries of atleast one of said first and said second alert envelopes.
 15. The methodof claim 14 wherein said step of outputting an alert signal furthercomprises the step of outputting a video control signal to controldisplay of terrain on a display device.
 16. The method of claim 14further comprising the step of defining a cut-off envelope to form aboundary of at least one of said first and second alert envelopes. 17.The method of claim 14 further comprising the step of receiving a firstand a second altitude signal from a distinct first and second sourcesrespectively to obtain a compound altitude signal representative of theaircraft altitude.
 18. The method of claim 14 wherein said step ofoutputting an alert signal comprises outputting an audio control signalto generate an aural alert.
 19. The method of claim 14 furthercomprising the step of outputting a video control signal to controlrepresentation of terrain in a first color for terrain located more thana predefined amount relative to current altitude of the aircraft and ina second color for terrain located less than said predefined amountrelative to said current altitude wherein said predefined amount is afirst value for a cruise phase of flight, a second value for an approachphase of flight, and a third value for a landing phase of flight.
 20. Acomputer program product for alerting a pilot of a rotary wing aircraftof proximity to terrain comprising: a computer readable storage mediumhaving computer readable program code means embodied in said medium,said computer readable program code means comprising: a first computerinstruction means for accessing a database of terrain information; asecond computer instruction means for accessing signals representativeof a position of the aircraft, a flight path angle of the aircraft and aspeed of the aircraft; a third computer instruction means for defining alook ahead distance; a fourth computer instruction means for defining afirst alert envelope, indicative of a first severity of terrain threat,wherein boundaries of said first alert envelope are determined as afirst function of the flight path angle, said look ahead distance, and aterrain floor boundary; a fifth computer instruction means for defininga second alert envelope, indicative of a second severity of terrainthreat, wherein boundaries of said second alert envelope are determinedas a second function of the flight path angle, said look ahead distanceand said terrain floor boundary; a sixth computer instruction means fordefining said terrain floor boundary as a function of an aircraftaltitude and a said speed; and a seventh computer instruction means foroutputting an alert signal when a subset of the stored terraininformation is located within the boundaries of at least one of saidfirst and said second alert envelopes.
 21. The computer program productof claim 20 further comprising an eighth computer instruction means foroutputting a video control signal to control display of terrain on adisplay device.
 22. The computer program product of claim 20 furthercomprising an eighth computer instruction means for defining a cut-offenvelope to form a boundary of at least one of said first and secondalert envelopes.
 23. The computer program product of claim 20 furthercomprising an eighth computer instruction means for accessing a firstand a second altitude signal from a distinct first and second sourcesrespectively to obtain a compound altitude signal representative of theaircraft altitude.
 24. The computer program product of claim 20 whereinsaid seventh computer instruction means further comprises a means foroutputting an audio control signal to generate an aural alert.
 25. Thecomputer program product of claim 20 further comprising an eighthcomputer instruction means for outputting a video control signal tocontrol representation of terrain in a first color for terrain locatedmore than a predefined amount relative to a current altitude of theaircraft and in a second color for terrain located less than saidpredefined amount relative to said current altitude wherein saidpredefined amount is a first value for a cruise phase of flight, asecond value for an approach phase of flight, and a third value for alanding phase of flight.
 26. An apparatus for alerting a pilot of ahover-capable aircraft of proximity to terrain, the apparatuscomprising: an input for receiving signals representative of a positionof the aircraft, a flight path angle of the aircraft and a speed of theaircraft, and coupled to a database of stored terrain information; anoutput; a signal processing device, coupled to said input, and coupledto said output, for: (a) defining a look ahead distance as a function ofa distance to transition from a first phase of flight to a hover phaseof flight; (b) defining a first alert envelope, indicative of a firstseverity of terrain threat, wherein boundaries of said first alertenvelope are determined as a first function of the flight path angle,said look ahead distance, and a terrain floor boundary; (c) defining asecond alert envelope, indicative of a second severity of terrainthreat, wherein boundaries of said second alert envelope are determinedas a second function of the flight path angle, said look ahead distanceand said terrain floor boundary; and (d) outputting an alert signal whena subset of the stored terrain information is located within theboundaries of at least one of said first and said second alertenvelopes.
 27. The apparatus of claim 26 wherein at least one of saidfirst and second alert envelopes is further bounded by a cut-offenvelope.
 28. The apparatus of claim 26 wherein said signalsrepresentative of the position of an aircraft include a first signalreceived from a satellite navigation system indicative of an aircraftaltitude and a second signal representative of the aircraft altitudereceived from a source other than the satellite navigation system, andwherein said signal processing device further comprises a means fordetermining a compound altitude signal.
 29. The apparatus of claim 26wherein the boundaries of at least one of said first and second alertenvelopes is further determined as a function of a configurable datum.30. The apparatus of claim 26 wherein at least one of said first andsecond alert envelopes further comprises a subset of alert envelopesrepresenting various severities of hazard to the aircraft.
 31. Theapparatus of claim 26 wherein said signal processing device comprises amicroprocessor.
 32. The apparatus of claim 26 wherein said signalprocessing device comprises a means for outputting said alert signal asa video control signal, wherein said video control signal is useful forcontrolling representation of terrain on a video display in variouscolors according to a degree of terrain threat.
 33. The apparatus ofclaim 26 further comprising a voice warning generator coupled to saidsignal processor and wherein said alert signal output from said signalprocessing device comprises an audio control signal to command saidvoice warning generator to output an aural alert.
 34. The apparatus ofclaim 26 wherein said speed comprises a groundspeed of the aircraft. 35.The apparatus of claim 26 wherein the aircraft is an airship.
 36. Theapparatus of claim 26 wherein the aircraft is a tilt rotor.
 37. Theapparatus of claim 26 wherein said signal processing device furthercomprises a means for outputting a video control signal to controlrepresentation of a background terrain data proximate the aircraft: in afirst color for terrain located more than a predetermined amountrelative to a current altitude of the aircraft wherein saidpredetermined amount is a first value for a cruise phase of flight and asecond value for an approach phase of flight and a third value for alanding phase of flight; and in a second color for terrain located lessthan said predetermined amount relative to said current altitude. 38.The apparatus of claim 26 wherein said cruise, approach and landingphases are defined as a function of said speed of the aircraft.
 39. Amethod for alerting a pilot of a hover-capable aircraft of proximity toterrain comprising the steps of: accessing a database of terraininformation; receiving signals representative of a position of theaircraft, a flight path angle of the aircraft and a speed of theaircraft; defining a look ahead distance as a function of a distance totransition from a first phase of flight to a hover phase of flight;defining a first alert envelope, indicative of a first severity ofterrain threat, wherein boundaries of said first alert envelope aredetermined as a first function of the flight path angle, said look aheaddistance, and a terrain floor boundary; defining a second alertenvelope, indicative of a second severity of terrain threat, whereinboundaries of said second alert envelope are determined as a secondfunction of the flight path angle, said look ahead distance and saidterrain floor boundary; and outputting an alert signal when a subset ofthe stored terrain information is located within the boundaries of atleast one of said first and said second alert envelopes.
 40. The methodof claim 39 wherein said step of outputting an alert signal furthercomprises the step of outputting a video control signal to controldisplay of terrain on a display device.
 41. The method of claim 39further comprising the step of defining a cut-off envelope to form aboundary of at least one of said first and second alert envelopes. 42.The method of claim 39 further comprising the step of receiving a firstand a second altitude signal from a distinct first and second sourcesrespectively to obtain a compound altitude signal representative of theaircraft altitude.
 43. The method of claim 39 wherein said step ofoutputting an alert signal comprises outputting an audio control signalto generate an aural alert.
 44. The method of claim 39 furthercomprising the step of outputting a video control signal to controlrepresentation of terrain in a first color for terrain located more thana predefined amount relative to a current altitude of the aircraft andin a second color for terrain located less than said predefined amountrelative to said current altitude wherein said predefined amount is afirst value for a cruise phase of flight, a second value for an approachphase of flight, and a third value for a landing phase of flight.
 45. Acomputer program product for alerting a pilot of a hover-capableaircraft of proximity to terrain comprising: a computer readable storagemedium having computer readable program code means embodied in saidmedium, said computer readable program code means comprising: a firstcomputer instruction means for accessing a database of terraininformation; a second computer instruction means for accessing signalsrepresentative of a position of the aircraft, a flight path angle of theaircraft and a speed of the aircraft; a third computer instruction meansfor defining a look ahead distance as a function of a distance totransition from a first phase of flight to a hover phase of flight; afourth computer instruction means for defining a first alert envelope,indicative of a first severity of terrain threat, wherein boundaries ofsaid first alert envelope are determined as a first function of theflight path angle, said look ahead distance, and a terrain floorboundary; a fifth computer instruction means for defining a second alertenvelope, indicative of a second severity of terrain threat, whereinboundaries of said second alert envelope are determined as a secondfunction of the flight path angle, said look ahead distance and saidterrain floor boundary; and a sixth computer instruction means foroutputting an alert signal when a subset of the stored terraininformation is located within the boundaries of at least one of saidfirst and said second alert envelopes.
 46. The computer program productof claim 45 further comprising a seventh computer instruction means foroutputting a video control signal to control display of terrain on adisplay device.
 47. The computer program product of claim 45 furthercomprising a seventh computer instruction means for defining a cut-offenvelope to form a boundary of at least one of said first and secondalert envelopes.
 48. The computer program product of claim 45 furthercomprising a seventh computer instruction means for accessing a firstand a second altitude signal from a distinct first and second sourcesrespectively to obtain a compound altitude signal representative of theaircraft altitude.
 49. The computer program product of claim 45 whereinsaid sixth computer instruction means further comprises a means foroutputting an audio control signal to generate an aural alert.
 50. Thecomputer program product of claim 45 further comprising a seventhcomputer instruction means for outputting a video control signal tocontrol representation of terrain in a first color for terrain locatedmore than a predefined amount relative to a current altitude of theaircraft and in a second color for terrain located less than saidpredefined amount relative to said current altitude wherein saidpredefined amount is a first value for a cruise phase of flight, asecond value for an approach phase of flight, and a third value for alanding phase of flight.
 51. An apparatus for alerting a pilot of arotary wing aircraft of proximity to terrain comprising: an input forreceiving signals representative of a position of the aircraft, a flightpath angle of the aircraft and a speed of the aircraft, and coupled to adatabase of stored terrain information; an output; and a signalprocessor, coupled to said input and to said output for: (a) defining alook ahead/look down alert envelope, wherein boundaries of said alertenvelope are determined as a function of the flight path angle, a lookahead distance, and a terrain floor boundary; wherein said terrain floorboundary comprises a function of an aircraft altitude and said speed,and wherein said look ahead distance comprises a function of a distanceto transition from a first phase of flight to a hover phase of flight;and (b) outputting an alert signal when a subset of the stored terraininformation is located within the boundaries of said alert envelope. 52.The apparatus of claim 51 wherein said look ahead/look down alertenvelope further comprises a first, caution, envelope and a second,warning, envelope.
 53. The apparatus of claim 52 wherein said signalprocessor outputs a first alert signal when said subset of the storedterrain information is located within the boundaries of said cautionenvelope and a second alert signal when said subset of the storedterrain information is located within the boundaries of said warningenvelope.
 54. The apparatus of claim 51 wherein said signal processorcomprises a microprocessor.
 55. The apparatus of claim 51 wherein saidspeed comprises a groundspeed of the aircraft.
 56. The apparatus ofclaim 51 wherein said signal processing device comprises a means foroutputting said alert signal as a video control signal, wherein saidvideo control signal is useful for controlling representation of terrainon a video display in various colors according to a degree of terrainthreat.
 57. The apparatus of claim 51 further comprising a voice warninggenerator coupled to said signal processor and wherein said alert signaloutput from said signal processing device comprises an audio controlsignal to command said voice warning generator to output an aural alert.58. The apparatus of claim 51 wherein said signal processing devicefurther comprises a means for outputting a video control signal tocontrol representation of a background terrain data proximate theaircraft: in a first color for terrain located more than a predeterminedamount relative to a current altitude of the aircraft wherein saidpredetermined amount is a first value for a cruise phase of flight and asecond value for an approach phase of flight and a third value for alanding phase of flight; and in a second color for terrain located lessthan said predetermined amount relative to said current altitude. 59.The apparatus of claim 58 wherein said cruise, approach and landingphases are defined as a function of said speed of the aircraft.
 60. Theapparatus of claim 51 wherein said signal processor further defines alook up envelope and outputs said alert signal when said subset ofterrain is located within said look up envelope.
 61. A method foralerting a pilot of a rotary wing aircraft of proximity to terraincomprising the steps of: receiving signals representative of a positionof the aircraft, a flight path angle of the aircraft and a speed of theaircraft, and stored terrain information; defining a look ahead/lookdown alert envelope, wherein boundaries of said alert envelope aredetermined as a function of the flight path angle, a look aheaddistance, and a terrain floor boundary; wherein said terrain floorboundary comprises a function of an aircraft altitude and a said speed,and wherein said look ahead distance comprises a function of a distanceto transition from a first phase of flight to a hover phase of flight;and outputting an alert signal when a subset of the stored terraininformation is located within said alert envelope.
 62. The method ofclaim 61 wherein said look ahead/look down alert envelope furthercomprises a first caution envelope and a second warning envelope. 63.The method of claim 62 further comprising the steps of outputting afirst alert signal when said subset of the stored terrain information islocated within the boundaries of said caution envelope and outputting asecond alert signal when said subset of the stored terrain informationis located within the boundaries of said warning envelope.
 64. Themethod of claim 61 further comprising the step of outputting a videocontrol signal, wherein said video control signal is useful forcontrolling representation of terrain on a video display in variouscolors according to a degree of terrain threat.
 65. The method of claim61 further comprising the step of outputting an aural alert.
 66. Themethod of claim 61 further comprising the step of outputting a videocontrol signal to control representation on a display of a backgroundterrain data proximate the aircraft: in a first color for terrainlocated more than a predetermined amount relative to a current altitudeof the aircraft wherein said predetermined amount is a first value for acruise phase of flight and a second value for an approach phase offlight and a third value for a landing phase of flight; and in a secondcolor for terrain located less than said predetermined amount relativeto said current altitude.
 67. The method of claim 66 further comprisingthe step of defining said cruise, approach and landing phases as afunction of said speed of the aircraft.
 68. The method of claim 61further comprising the step of defining a look up envelope andoutputting said alert signal when said subset of terrain is locatedwithin said look up envelope.
 69. A computer program product foralerting a pilot of a rotary wing aircraft of proximity to terraincomprising: a computer readable storage medium having computer readableprogram code means embodied in said medium, said computer readableprogram code means comprising: a first computer instruction means foraccessing signals representative of a position of the aircraft, a flightpath angle of the aircraft and a speed of the aircraft, and storedterrain information; a second computer instruction means for defining alook ahead/look down alert envelope, wherein boundaries of said alertenvelope are determined as a function of the flight path angle, a lookahead distance, and a terrain floor boundary; wherein said terrain floorboundary comprises a function of an aircraft altitude and said speed,and wherein said look ahead distance comprises a function of a distanceto transition from a first phase of flight to a hover phase of flight;and a third computer instruction means for outputting an alert signalwhen a subset of the stored terrain information is located within saidalert envelope.
 70. The computer program product of claim 69 whereinsaid second computer instruction means further defines said lookahead/look down alert envelope as comprising a first caution envelopeand a second warning envelope.
 71. The computer program product of claim70 further comprising a fourth computer instruction means for outputtinga first alert signal when said subset of the stored terrain informationis located within the boundaries of said caution envelope and outputtinga second alert signal when said subset of the stored terrain informationis located within the boundaries of said warning envelope.
 72. Thecomputer program product of claim 69 further comprising a fourthcomputer instruction means for outputting a video control signal,wherein said video control signal is useful for controllingrepresentation of terrain on a video display in various colors accordingto a degree of terrain threat.
 73. The computer program product of claim69 further comprising a fourth computer instruction means for outputtingan aural alert.
 74. The computer program product of claim 69 furthercomprising a fourth computer instruction means for outputting a videocontrol signal to control representation on a display of a backgroundterrain data proximate the aircraft: in a first color for terrainlocated more than a predetermined amount relative to a current altitudeof the aircraft wherein said predetermined amount is a first value for acruise phase of flight and a second value for an approach phase offlight and a third value for a landing phase of flight; and in a secondcolor for terrain located less than said predetermined amount relativeto said current altitude.
 75. The computer program product of claim 74further comprising a fifth computer instruction means for defining saidcruise, approach and landing phases as a function of said speed of theaircraft.
 76. The computer program product of claim 69 furthercomprising a fourth computer instruction means for defining a look upenvelope and wherein said third computer instruction means outputs saidalert signal when said subset of terrain is located within said look upenvelope.
 77. A ground proximity warning system for rotary wing aircraftcomprising: a warning computer including: (a) an input for receivingsignals representative of a position of the aircraft, a flight pathangle of the aircraft and a speed of the aircraft, and coupled to adatabase of stored terrain information; (b) an output; and (c) a signalprocessor, coupled to said input and to said output for: (i) defining analert envelope, wherein boundaries of said alert envelope are determinedas a function of the flight path angle, a look ahead distance, and aterrain floor boundary; wherein said terrain floor boundary comprises afunction of an aircraft altitude and said speed, and wherein said lookahead distance comprises a function of a distance to transition from afirst phase of flight to a hover phase of flight; and (ii) outputting analert signal when a subset of the stored terrain information is locatedwithin the boundaries of said alert envelope; and a display, having andisplay input coupled to said output of said warning computer, fordisplaying said terrain data proximate the aircraft in various colorsaccording to a degree of terrain threat.
 78. The system of claim 77wherein said warning computer comprises a general purpose processor. 79.The system of claim 77 wherein said speed comprises a groundspeed of theaircraft.
 80. The system of claim 77 wherein the aircraft is a tiltrotor.
 81. A ground proximity warning system for rotary wing aircraftcomprising: a warning computer including: (a) an input for receivingsignals representative of a position of the aircraft, a flight pathangle of the aircraft and a speed of the aircraft, and coupled to adatabase of stored terrain information; (b) an output; and (c) a signalprocessor, coupled to said input and to said output for: (i) defining analert envelope, wherein boundaries of said alert envelope are determinedas a function of the flight path angle, a look ahead distance, and aterrain floor boundary; wherein said terrain floor boundary comprises afunction of an aircraft altitude and said speed, and wherein said lookahead distance comprises a function of a distance to transition from afirst phase of flight to a hover phase of flight; and (ii) outputting analert signal when a subset of the stored terrain information is locatedwithin the boundaries of said alert envelope; and a display, having andisplay input coupled to said output of said warning computer, for: (a)displaying said terrain data located in the boundaries of said alertenvelope in various colors according to a degree of terrain threat; and(b) displaying terrain data proximate the aircraft: (i) in a first colorfor terrain located more than a predetermined amount relative to acurrent altitude of the aircraft wherein said predetermined amount is afirst value for a cruise phase of flight and a second value for anapproach phase of flight and a third value for a landing phase offlight; and (ii) in a second color for terrain located less than saidpredetermined amount relative to said current altitude.
 82. The systemof claim 77 wherein the aircraft is a tilt rotor.
 83. The method ofclaim 14 wherein said speed comprises a groundspeed of the aircraft. 84.The computer program product of claim 20 wherein said speed comprises agroundspeed of the aircraft.
 85. The method of claim 39 wherein saidspeed comprises a groundspeed of the aircraft.
 86. The computer programproduct of claim 45 wherein said speed comprises a groundspeed of theaircraft.
 87. The method of claim 61 wherein said speed comprises agroundspeed of the aircraft.
 88. The computer program product of claim69 wherein said speed comprises a groundspeed of the aircraft.
 89. Themethod of claim 1 wherein said look ahead distance comprises a functionof a distance to a nearest runway.